Which component is NOT included in the transcriptional unit?

A transcriptional unit encompasses the essential components involved in the process of transcription, where the DNA sequence is copied into RNA. This unit typically consists of the promoter, which serves as a binding site for RNA polymerase to initiate transcription, the RNA coding region, which contains the information that will be transcribed into RNA, and the terminator, which signals the end of transcription.

The translation initiation site, however, is not part of the transcriptional unit. Instead, it is relevant to the process of translation, where messenger RNA (mRNA) is used as a template to synthesize proteins. The translation initiation site is where the ribosome assembles to begin translating the mRNA into a polypeptide chain. Therefore, this component is integral to the translation process, not the transcriptional unit itself. This distinction makes it clear why the translation initiation site is the correct choice for what is not included in a transcriptional unit.